This invention relates to optical fibre reflectometers, and is particularly concerned with avoiding the problems of excessive reflection at the interface between the reflectometer and the optical fibre to be tested.
In an optical fibre reflectometer light from a source, typically an injection laser, is directed through a multi-port optical fibre coupler into the optical fibre under test. Some of the light that is backscattered in the test fibre returns to the coupler, and some of this returning light is directed into a branch of the coupler terminated with a photodetector.
For testing multimode fibre a particularly satisfactory type of coupler is one in which a vapour deposited silica (VDS) fibre (possessing a waveguiding structure within the glass), with a low index plastics coating surrounding the glass, is optically coupled in a side-by-side relationship with a plastics-clad-silica (PCS) fibre. Such a coupler is described in U.S. Pat. No. 4,474,431. When used in a reflectometer it is arranged that light from the source is launched into the core of one end of the VDS fibre, the test fibre is optically coupled with the other end of the VDS fibre, and the photodetector is optically coupled with the end of the PCS fibre adjacent the end of the VDS fibre to which the source is coupled. The cross-section of the core of the VDS fibre is chosen to be effectively smaller than that of the test fibre, while its cladding is preferably matched in size with that of the test fibre. The coupling between modes propagating in the PCS fibre and core modes of VDS fibre is very small, but the coupling between the PCS fibre and cladding modes of the VDS fibre (modes guided by the glass/plastics interface of the VDS fibre) is relatively strong. Light from the source is launched into the core of the VDS fibre of the coupler, and therefore passes through the coupler with very little attenuation. Any light backscattered in the test fibre that reaches the coupler propagating in test fibre cladding modes is launched into cladding modes of the coupler VDS fibre, where it is strongly coupled into the PCS fibre, and hence is directed to the photodetector with relatively high efficiency. Some of any light backscattered in the test fibre that reaches the coupler propagating in test fibre core modes is launched into core modes of the coupler VDS fibre. Most of this light passes straight through the coupler, and hence little reaches the photodetector. On the other hand, since the core of the coupler VDS fibre is effectively smaller than the core of the test fibre, much of the backscattered light that reaches the coupler propagating in test fibre core modes is not launched into core modes of the VDS fibre, but is launched into its cladding. This light is strongly coupled to the PCS fibre, and hence much of it reaches the detector. Under appropriate conditions the attenuation afforded by the coupler of light suffering a double pass through the coupler, finally to emerge from a different port from that by which it first entered, can be significantly less than the theoretical 6 dB minimum of a conventional biconical coupler as described for instance by B. S. Kawasaki and K. O. Hill in a paper entitled `Low-loss access coupler for multimode optical fibre distribution networks`, (Applied Optics Vol. 16 No. 7 pp 1794-5, July 1977).
The way that the test fibre is coupled to the coupler typically involves ferrule terminating both the VDS fibre of the coupler and the test fibre, and then butting the ferrules together while they are held in alignment in a Vee-groove. The termination on the VDS fibre of the coupler is, in the course of normal use, butted up many times against a large number of different test fibres. In the course of this use the end of the VDS fibre is prone to cumulative damage by scratching. Such scratching may be the result of the trapping of dirt between the mating surfaces, or because test fibres have been connected that have protruded slightly from the ends of their ferrules. To reduce this risk it has been the practice to provide the VDS fibre termination with a sapphire window which is less susceptible to scratching on account of its relative hardness.
The difference between the refractive index of sapphire and that of the optical fibres means that partial reflection occurs at the interface. Virtually all the light that is not launched into the test fibre, but is reflected back into the VDS fibre by the fibre/sapphire interfaces, is light reflected back to propagate in core modes of the VDS fibre. This is because such light was propagating in core modes before reflection, and the sapphire window is normal to the fibre axis. Thus virtually none of this light reflected by these interfaces is coupled into the PCS fibre which would direct it to the photodetector. It is seen therefore that the deleterious effect of such reflections is substantially entirely limited to the effect of the attenuation of power launched into the test fibre.
The above described coupler employing VDS and PCS fibres, while well-suited for use in reflectometers designed solely for making measurements on multimode fibres, is not best suited where is facility for making measurements on single mode fibres is additionally required. The reason for this is that the satisfactory operation of the coupler requires the core of the VDS fibre of the coupler to be significantly smaller in effective cross-section than that of the test fibre so that a significant proportion of the backscattered light will be launched, not into core modes of the VDS fibre, but into its cladding modes. It is not possible to construct a VDS fibre to have a core with an effective cross-section significantly smaller than that of a conventional single mode fibre. Therefore, when a single mode fibre testing facility for a reflectometer is required, recourse is typically had to a biconical tapered coupler (single mode or multimode) of the type previously referred to. Such a coupler is preferably made by the method described in United Kingdom Patent Specification No. 2150703. This substitution of coupler type leaves unchanged the problem of cummulative damage to the end of the fibre of the coupler to which the test fibres have to be connected. Now however the problem of refractive index mismatch reflections occasioned by the use of a sapphire window are potentially much more serious. This is because the coupler will couple substantially half the reflected power into the fibre to which the photodetector is connected. Unless special precautions are taken, it is prone to swamp the photodetector to the extent that it is unable to respond to the backscattered light that reaches it immediately after it receives the light reflected by the fibre/sapphire interfaces. This problem can be resolved by optical range-gating, for instance with a Bragg acousto-optical deflector, the optical signal before it reaches the photodetector. This is the approach of various workers in the field of optical time domain reflectometry, being described for instance by M. P. Gold and A. H. Hartog in a paper entitled `A practical high performance single mode OTDR system for the long-wavelength region`, (9th European Conference on Optical Communications, September 1983), and by M. Nakazawa et al in a paper entitled `Marked extension of diagnosis length in Optical Time Domain Reflectometry using a 1.32 um YAG Laser` (Electronics Letters, Oct. 15, 1981 Vol. 17 No. 2 pp 783-4).
The present invention is concerned with a design of reflectometer in which the need for optical range-gating is circumvented.